Among catalysts for NOx removal (DeNOx catalysts) used for purification of NOx in exhaust gas emitted from diesel engines, in-cylinder gasoline direct injection engines (GDI), and the like, there is a NOx occlusion/reduction type catalyst called a lean NOx trap (LNT). This catalyst is formed by supporting an occlusion material such as an alkali metal (for example, potassium K or the like) or an alkaline earth metal (for example, barium Ba or the like) together with a noble metal such as platinum Pt.
In an air-fuel ratio lean state where exhaust gas is oxygen-rich, the NOx occlusion/reduction type catalyst oxidizes NO in the exhaust gas, and occludes NOx in the form of a nitrate salt of the occlusion material. Meanwhile, in an air-fuel ratio rich state where the exhaust gas contains almost no oxygen, the catalyst releases the occluded NOx, and reduces the released NOx with a reducing agent such as hydrocarbons (“HCs”) or CO by way of a three-way catalyst function. By way of these functions, the catalyst reduces the amount of NOx.
In regeneration control by NOx removal for restoring the NOx occlusion capacity of the NOx occlusion/reduction type catalyst, when the air-fuel ratio of exhaust gas is brought into a rich state, a reducing agent such as fuel is supplied to the NOx occlusion/reduction type catalyst. The supply of the reducing agent is achieved by performing post-injection in which fuel is additionally injected after main injection during in-cylinder fuel injection control, or by performing in-exhaust pipe direct injection in which fuel is injected directly into an exhaust pipe.
In addition, there are continuous regeneration-type diesel particulate filter devices (DPF devices) for collecting PMs (particulate matters) emitted from diesel engines with filters. In such a continuous regeneration DPF device, PMs collected on the filter are continuously combusted for purification when the temperature of exhaust gas is relatively high (approximately 350° C. or above). When, however, the temperature of the exhaust gas is low, the temperature of an oxidation catalyst or a PM oxidation catalyst supported on the filter is lowered, and thus the oxidation catalyst or the PM oxidation catalyst is not activated. This makes it difficult to self-regenerate the filter through oxidation of the PMs. For this reason, clogging due to deposition of PMs on the filter progresses, causing a problem of exhaust pressure rise.
In this respect, once the amount of PMs deposited on the filter exceeds a predetermined amount, forced regeneration by PM removal is performed. In the forced regeneration by PM removal, the temperature of exhaust gas is forcibly raised, and the collected PMs are removed by forced combustion. In the forced regeneration by PM removal, post injection or in-exhaust pipe direct injection is performed to supply unburned HCs (hydrocarbons) such as a fuel into the exhaust gas, and the supplied unburned HCs are combusted on an oxidation catalyst disposed on the upstream side of the filter or on an oxidation catalyst supported on the filter. By utilizing the heat of the oxidation reaction, the temperatures of exhaust gas at an inlet of the filter and on a surface of the filter are raised. Thereby, the temperature of the filter is raised to a temperature not lower than a temperature at which PMs accumulated on the filter are combusted, and thus PMs are removed by combustion.
There exist post injection and in-exhaust pipe direct injection as methods for supplying unburned HCs into an exhaust pipe as described above. The in-exhaust pipe direct injection is advantageous in that the supply amount of unburned HCs can be adjusted without affecting the combustion in a cylinder. Hence, the in-exhaust pipe direct injection in which fuel is directly injected into an exhaust pipe is being put into practical use for injecting a reducing agent for a NOx occlusion/reduction type catalyst, or for raising the temperature of exhaust gas for the purpose of forcibly combusting PMs collected on a DPF.
However, in a case of the in-exhaust pipe direct injection, even when fuel is injected into an exhaust pipe, no oxidation reaction occurs at a temperature which is not higher than an activation temperature of a NOx occlusion/reduction type catalyst and an oxidation catalyst, as in the case of the post injection. The injected fuel passes through these catalysts, and outflows, causing white smoke and the like. In such a case, since the unburned HCs are not gasified in a case of the in-exhaust pipe direct injection, unlike a case of the post injection, white smoke is more likely to be caused in the case of the in-exhaust pipe direct injection than in a case of the post injection. For this reason, the range where the in-exhaust pipe direct injection can be used is limited by the activation temperature of the catalyst, and there is a problem that it is difficult to perform regeneration of a NOx occlusion/reduction type catalyst by NOx removal or forced regeneration of a DPF by PM removal during low-load operation or the like.
Meanwhile, there has been proposed a cell structure as descried in, for example, Japanese patent application Kokai publication No. 2005-199179. The cell structure has an outer wall portion on an outer peripheral surface and a cell structural portion having partition walls forming multiple cells extending from one end face to the other end face inside the outer wall portion. In the cell structure, the cell structural portion is mainly composed of a ceramic material or a metallic material, and the outer wall portion is mainly composed of a ceramic material, so that a heat insulating performance of the outer wall portion is enhanced. This improves the rate of the temperature rise of the cell structural portion, and makes the temperature distribution uniform.